This invention relates to an improved continuous adsorption process for the resolution of hydrocarbon mixtures into products of like molecular structure. More particularly, this process relates to the application of multiple molecular sieve adsorbent beds to the separation of normal paraffins from a vapor-phase hydrocarbon mixture containing the same.
It is recognized that resolution of the components of certain fluid solutions can be achieved through exploitation of the adsorptive properties of materials commonly known as molecular sieves. Such materials, principally the natural and synthetic aluminosilicates, have a porous crystalline structure with intracrystal cavities that are accessible via pores of relatively uniform diameter. Adsorption through the pores is selective--only molecules with an effective diameter smaller than the characteristic pore diameter of a particular molecular sieve can be adsorbed thereby. Thus, a basis is provided for separation of molecules according to size. Molecular sieves are particularly useful for accomplishing the separations of mixtures of hydrocarbons of differing molecular structures, for instance the separation of normal paraffins from mixtures also comprising branched and/or cyclic hydrocarbons, which separations are not generally feasible through more common techniques such as fractional distillation or solvent extraction.
In the application of a molecular sieve to such separations, a mixed feedstock is passed over a contained bed of the sieve material to accomplish adsorption thereon of selected molecules, termed the adsorbate fraction of the feedstock. Effluent from the bed comprises the remaining fraction of the feedstock, herein termed the raffinate. Adsorption is, of course, but one phase of the overall separation process, since the adsorbate must eventually be desorbed from the sieve. One common method for accomplishing such desorption involves discontinuing the flow of feedstock and passing a stream of an eluent over the bed. The eluent is generally a compound which is itself adsorbed through the sieve pores. For instance, when the adsorbate is a normal paraffin of a given carbon number, a preferred eluent is a normal paraffin of a different carbon number. In this case both the adsorption and desorption phases of the overall separations process involve interchange of eluent and adsorbate molecules on the sieve bed--adsorbate molecules are displaced from the sieve pores by eluent molecules during the desorption step and eluent is displaced by adsorbate during a subsequent adsorption step. A mixture of raffinate and eluent molecules is withdrawn as effluent from the bed during adsorption service by the bed, and a mixture of adsorbate and eluent is withdrawn during desorption. Such effluent mixtures, respectively termed the process raffinate and adsorbate products, are generally then subjected to further processing for the recovery of eluent for recycle to the adsorption beds.
With respect to the use of a given sieve bed for separations purposes, the performance of distinct adsorption and desorption steps does not permit a continuous process as is often desired for efficient commercial operations. It is recognized, however, that certain discontinuities associated with the use of a single bed can be eliminated and other processing advantages realized through the use of multiple sieve beds.
In the context of vapor-phase adsorption processes for the separation of normal paraffins from hydrocarbon mixtures, one such multi-bed process which has proven to be of particular advantage is that of U.S. Pat. No. 3,451,924. Through repeated switching of process flows to three adsorbent beds in a 6 step sequence, the process of this patent achieves continuity with respect to the flow of both hydrocarbon feed and eluent to the beds. Furthermore, through series flow of certain process streams through two adsorbent beds, the process provides for loading of each adsorbent bed to near the full capacity without loss of the normal paraffins to the process raffinate product.
The prior art process of U.S. Pat. No. 3,451,924 can be more particularly described through reference to attached FIG. 1, which in six parts, labeled (a) through (f), illustrates schematically each of the six process steps. Referring to FIG. 1(a), depicted therein is a step of the process in which a continuous flow of a vapor-phase normal paraffin-containing mixed hydrocarbon feed stream designated 10 is passed to a first sieve bed designated A which functions as a primary adsorption bed to adsorb said feed normal paraffins. Effluent, stream 11, is withdrawn from bed A and passed to another bed labeled B which serves as a secondary adsorption bed, capturing normal paraffins which escape adsorption in, or "break through", sieve bed A. A process raffinate product, stream 20, composed primarily of non-normal-paraffin hydrocarbons from the feed and of eluent, is withdrawn from bed B. This raffinate mixture is typically separated into an eluent fraction and a non-normal-paraffin hydrocarbon fraction by downstream processing facilities not a part of the adsorption process and not here shown. The separated eluent fraction is usually recycled. Also during the process step depicted in FIG. 1(a), a continuous flow of eluent 30 is passed to a previously loaded bed C for desorption of normal paraffins therein. A process adsorbate product 40 is withdrawn from bed C. This adsorbate product is then typically separated into a feed normal paraffin fraction and an eluent fraction by downstream processing facilities not shown, and the eluent recycled to the adsorption process.
The prior art process step depicted in FIG. 1(a) is continued until bed A is loaded to substantially full capacity with adsorbate and desorption of bed C is essentially complete, at which time process flows are switched to the step of FIG. 1(b). Now, referring to this Figure, the continuous flow of hydrocarbon feed, again designated 10, is passed directly to sieve bed B which serves as a sole adsorption bed for this process step. The continuous eluent flow 30 is passed to bed A to purge non-adsorbed feed hydrocarbons from the void spaces therein. Since the purge effluent stream 31 from purge bed A contains quantities of unadsorbed and desorbed normal paraffins, it is passed to freshly desorbed bed C which serves as a purge guard bed wherein these normal paraffins can be captured. Effluent from bed B and effluent from bed C, both composed substantially of feed non-normal-paraffin hydrocarbons and eluent, may be combined as shown in a single raffinate product 20. Alternatively, the two effluent streams may be maintained as separate raffinate products for downstream use or processing. There is no process adsorbate product stream during the process step of FIG. 1( b).
Once bed A has been effectively purged of non-normal-paraffin hydrocarbons, process flows are switched to the step illustrated in FIG. 1(c). This step is in principle very similar to that of FiG. 1(a), as is indicated by process stream designations common to the two figures. Here, however, bed A is the desorption bed, bed B is the primary adsorption bed, and bed C is the secondary adsorption bed. The process is in turn switched to the steps of FIGS. 1(d), 1(e), and 1(f). Upon completion of the step of FIG. 1(f), the process is switched to that of FIG. 1(a). The six step process sequence is continuously repeated in this manner as many times as is desired. The service of each bed in each of the six process steps is summarized in Table I:
TABLE I ______________________________________ The step of: bed A bed B bed C ______________________________________ FIG. 1(a) primary secondary desorption adsorption adsorption FIG. 1(b) purge sole purge adsorption guard FIG. 1(c) desorption primary secondary adsorption adsorption FIG. 1(d) purge purge sole guard adsorption FIG. 1(e) secondary desorption primary adsorption adsorption FIG. 1(f) sole purge purge adsorption guard ______________________________________
In view of the continuous cyclic nature of this process, it has been termed the "Merry-Go-Round" process.
Despite the commercial success which the process of U.S. Pat. No. 3,451,924 has enjoyed, there are a number of disadvantages associated with its operation and performance. For instance, it is observed through reference to FIG. 1 that there is no process adsorbate product stream during three of the six process steps. In the process steps depicted in FIGS. 1(a), 1(c) and 1(e), there is a process raffinate product 20 which closely corresponds in mass flowrate to the hydrocarbon feed. In addition, there is also during these three steps, a process adsorbate product 40 which closely corresponds in mass flowrate to the eluent stream. However, in the steps of FIGS. 1(b), 1(d), and 1(f), there is only a raffinate product stream which corresponds in mass flowrate to the sum of that of the feed and eluent streams. Downstream processing of such vapor-phase product streams which are subject to repeated discontinuities in flowrate and composition has proven most difficult. For example, it has been impossible to implement efficient heat conservation measures and fully stable downstream processes for eluent recovery from adsorbate and raffinate product streams.
Furthermore, the lack of adsorbate product flow during the three steps of the prior art process can be viewed as a measure of process inefficiency with respect to optimizing the utilization of the sieve material. During these three steps, two of the three beds, and therefore two-thirds of the sieve adsorbent, is employed for purge and purge guard service rather than directly in adsorption for recovery of desired n-paraffins from a hydrocarbon feed.
Still further, the use of a freshly desorbed sieve bed for purge guard service in the prior art process steps of FIGS. 1(b), 1(d), and 1(f) has adverse affects upon the performance of this same bed in immediately subsequent adsorption service. The purge stream contains not only the non-normal-paraffin feed hydrocarbons that are being purged from the purge bed voids but also a considerable amount of feed normal paraffins which were eluted from the purge bed by the purge eluent flow. In the prior art process the feed normal paraffins are adsorbed from the purge effluent stream by the front part of the purge guard bed. However, the purge guard bed is next switched to secondary adsorption service, where the flow to the bed is for the most part a mixture of non-normal-paraffin feed hydrocarbons and eluent desorbed from the primary adsorption bed. The eluent in this flow tends to broaden the adsorption front in the secondary bed by desorbing feed normal paraffins from the front part of the bed which, in turn, are then readsorbed further downstream in the bed where the concentration of feed n-paraffins is lower. As a consequence, at the time the bed is switched from secondary adsorption to primary adsorption, the feed normal paraffins are not adsorbed in a sharp adsorption front near the inlet to the sieve bed, but instead are spread throughout the bed. When hydrocarbon feed is passed over the bed during its subsequent primary adsorption service, breakthrough of feed normal paraffins into the bed effluent is encountered well before the bed is loaded to the desired capacity.